The present invention relates to an add drop multiplexer (ADM) related to a synchronous multiplexing method and in particular, to an add drop multiplexer suitable for the case where it is required that the signal delay produced in the path line be small.
A prior art add drop multiplexer is disclosed in a report published in IEEE GLOBECOM' 86, No. 33.5.1-No. 33.5.5 pp. 1195-1199. In this multiplexer, as indicated in FIG. 1, signals in a high speed highway are branched to a low speed highway and signals in the low speed highway are inserted in the high speed high way. The add drop multiplexer in the literature stated above is applied to the 3rd order group of synchronous transmission lines in North America indicated in FIG. 2. The transmission lines are classified into 3 ranks, i.e. 1st order group, 2nd order group and 3rd order group, increasingly numbered with increasing bit rate, which are multiplexed at 1.544 Mb/s, 6.312 Mb/s an 45 Mb/s, respectively.
In prior art synchronous multiplexers there are disposed frame aligners in the receiving interface section in order to absorb fluctuations in phase (wander) having a long period in the received signal, accompanied by elongation and contraction of transmission lines due to fluctuations in temperature around the transmission lines and relays, and to match the phase of the received signal with the working phase in the multiplexer. In the case described in the above literature, in order to realize the functions stated above, an elastic store memory, (ES), which is a first-in-first-out memory is disposed as the frame aligner in the receiving interface section.
The add drop multiplexer is described in "Foreign Communication Techniques", December 1985, pp. 33-34, published by NTT.
According to the prior art techniques the frame aligner is disposed on the signal transmission line in the receiving interface section to absorb the wander produced on the signal transmission line and to effect the phase matching of the received signal with the working phase in the multiplexer. Consequently, a signal delay is added at the frame aligner section, until the signal received through the transmission line is sent to the transmission line as the pass-through line signal as it is.
FIG. 3 is a scheme for explaining the construction of a ring type transmission line, conceivable as a topological construction, to which a multiplexer according to the present invention is applied.
Among the nodes connecting a ring type transmission line transmitting signals only in one direction in FIG. 3, there are nodes 200-212, from which the transmission line goes out only in two directions, and nodes 100-102, from which the transmission line goes out in three directions to connect it with another ring type transmission line. The application of the add drop multiplexer according to the present invention is contemplated for the nodes 200-212, from which the transmission line goes out in two directions in these two kinds of nodes.
In FIG. 3, A and C represent ring type transmission lines in use and B and D represent spare ring type transmission lines, which are turned-on, when the transmission lines A and C are out of order, respectively, to be used instead thereof. When a signal is transmitted from the node 200 to the node 201, it is sent in the direction indicated by an arrow through the transmission line A. On the contrary, when the signal is transmitted from the node 201 to the node 200, it is sent through the nodes 202, 203, . . . , 206 and 100, making almost one turn.
In the ring type transmission line, as indicated in FIG. 3, since the signal delay due to the multiplexers installed in the nodes 200 to 212 is added to the signal propagation delay on the transmission line, even if it serves as a pass-through line, it is required to reduce the signal delay due to the multiplexer installed in each of the nodes. However, the add drop multiplexer according to the prior art techniques as described above has a problem that the signal delay due to the frame aligner inserted in series in the signal transmission line is predominant in the signal delay within the multiplexer and that it is so great that it cannot be neglected with respect to the signal propagation delay on the transmission line.